This is the closest thing I could find which addresses this;

OBR 8.01(a): The Windup Position... he may take one step backward, and one step forward with his free foot.

As long as he delivers the pitch to the plate, I think I would not balk it. There really is nothing that says during that step forward, his pivot foot can not lose contact with the rubber. And it is not even mentioned for the Set Position.

I was able to reproduce this action quite easily. Just push off the rubber with a little hop and you get it. The pitcher probably thinks he is getting another couple of MPH on his pitches not realizing that once you lose contact with the rubber, you lose the power of your legs and hips. It is actually a disadvantage for the pitcher because he loses his stabilizing point. My guess is this kid does not get the velocity he could and probably does not hit his spots.

But I don't see how he is gaining an advantage on the runners or deceiving them in any way ? I suggest he gets a new pitching coach also.

Whether a balk or not was not the question. And being able to reproduce it means nothing. It just means you would be called for illegal pitch also. He is getting closer to the plate before releasing the pitch, which is an advantage. If you don't stop this he will hop scotch to the grass in front of the mound.

Maybe not
 

We have one kid (going to local college) who did this move, and he was very precise with his control which surprised me.

I did notice though late in the season he had seemed to lose some velocity. Maybe because of his unusual mechanics.

But I asked him about it once, and he said he was taught that at a pitching camp - go figure.

Hey he threw upper 80's and threw strikes, my kind of pitcher.

thanks
David

As Garth said and I agree now that I'm seeing the correct picture, he's off pretty far and pretty soon in his delivery, compared to the phenomenon I was describing that applies to most pitchers. This is, indeed, an extreme motion, but I still don't know what I'd do about it.